286 



geological age based on terrestrial and freshwater faunas and floras only 

 are extremely likely to be incorrect. 



Unfortunately for us, the deposits in which we are now especially in- 

 terested contain few or no marine organisms, but abundant freshwater 

 and terrestrial animals and numerous plants. We must therefore reach 

 our conclusions by somewhat indirect methods and must be on our guard 

 against errors. Still more unfortunately for us, the paleozoologists and 

 the paleobotanists have not attained the same results from their studies. 



5. The Value of Plants as Indices of Geological Dates. 



I trust that the paleobotanists will not charge me with trying to dis- 

 parage their science when I proceed to show that, in the present case at 

 least, their results are less to be depended on than those obtained by the 

 paleozoologists. Without doubt, the plants have as interesting, as trust- 

 worthy, and as valuable a story to tell, when rightly deciphered, as do the 

 animals. It seems, however, that in some cases, other than the one before 

 us, the significance of fossil plants has not been rightly comprehended. In 

 Blanford's address, cited above, he mentions two important cases in which 

 the determination of the age of certain formations have contradicted those 

 made from the marine animals. One case is found in the Gondwana 

 system of India, where, as Blanford says, "we have a Rhaetic flora over- 

 lying a Jurassic flora and a Triassic fauna above both." Again he states 

 that "in Australia we find a Jurassic flora associated with a Carboniferous 

 marine fauna and overlain by a Permian freshwater fauna." 



The following is quoted from Lapparent (Traits, p. 71S) : 



A plus d'une reprise, l'etude des flores terrestres a paru donner des 

 indications contradietoires avec cedes des faunes marines; et en derniere 

 analyse la question a toujours ete tranchee en favour de ces dernieres. 



Geikie makes the following observation : 



Certainly a number of instances are known where an older type of 

 marine fauna is associated with a younger type of flora. 



One reason why plants, at least those of the northern hemisphere, 

 which have existed since the beginning of the Upper Cretaceous, seem to 

 be of only secondary value in correlating formations is found in their 

 apparently extreme conservatism. While the species have changed, the 

 genera have changed little. As an illustration of this, one may take the 

 list of plants published by Doctor Knowlton (Wash. Acad. Sci., xi, 1909, 



